In the typical air cycle refrigeration system employed to pressurize and cool aircraft cabins or similar loads, inlet or supply air obtained from a gas turbine engine compressor is pressurized in the refrigeration system compressor, cooled by ambient air in a heat exchanger and then fed to the system's turbine, work done on the turbine by the compressed air causing a cooling of the air which is then discharged to the load. The turbine is mechanically connected to the compressor such that work done on the turbine by the compressed air aids in driving the compressor rotor.
It is understood that the operation of a gas turbine engine compressor varies significantly with varying modes of engine operation such as during the flight of an aircraft powered by the engine. By way of example, in a cruise mode of operation, where considerably thrust is required, the engine's compressor passes much greater flows of air therethrough than, for example, in a decent mode of operation, where as little thrust as possible is desired. Accordingly, the flow of bleed air available from the engine's compressor for the operation of the aircraft's refrigeration system also varies significantly throughout the various modes of aircraft operation. To maintain a constand environment in the aircraft cabin for minimization of passenger discomfort, it is required that the air cycle refrigeration system accommodate such variations in bleed air supply thereto by adjustments in the flow capacity of the refrigeration system. Of course, such adjustments to the system's capacity should not be at the expense of the ability of the system to adequately cool and pressurize the load. Although this discussion has dealt with gas turbine engine applications in aircraft, it will be understood that such engine compressor bleed pressure variations due to variations in engine operation from idle to full power are exhibited in any of various other engine applications.
Various schemes have been proposed for the purpose of achieving wide ranges of system flow capacity of the refrigeration system to accommodate expected variations in supply pressure. In one such scheme, the system's compressor discharges to the turbine through a variable area turbine inlet nozzle. The nozzle is opened for increased flow capacity when supply pressure is low and closed to reduce the system's flow capacity when supply pressure is high. However, it has been found that opening the turbine inlet nozzle has only a slight effect on the compressor flow capacity thereby doing little to enhance air flow through the system. Furthermore, increasing the turbine nozzle area reduces the pressure rise associated with the compressor, thereby reducing the overall efficiency of the system. It has been found that such prior art systems utilizing a variable area turbine inlet nozzle are limited to about 25 percent variations in system flow capacity.
An alternate scheme for providing sufficient flow range involves provision of a variable area compressor discharge nozzle in combination with a variable area turbine inlet nozzle so that an adjustment in one of the nozzle areas to modulate the flow capacity of the system may be accompanied by an adjustment in the other nozzle area thereby matching the flow characteristics of the compressor with those of the turbine. However, a system of this type employing both variable turbine and compressor nozzles may be overly complex and costly for its intended application.
Still another prior art scheme to provide varying flow capacity employs the selective addition of system inlet air to the turbine discharge flow. While this flow addition to the turbine discharge may be effective for modulating flow (cabin pressurization), it achieves no additional cooling and, therefore, may be unsuitable under conditions of low engine bleed conditions when increased cooling capacity is required.
A last prior art arrangement for varying the flow capacity of an air cycle refrigeration machine includes the provision of a second air cycle system operated as a turbo-supercharger to increase the supply pressure into (and therefore, the flow through) the primary system's turbine for enhanced cooling capability. However, such an additional air cycle system or turbo-supercharger would of course, contribute substantially to the cost and complexity of the system.